The present invention relates to improvements in an electric power unit for electric discharge surface treatment, and also the present invention relates to improvements in a method of electric discharge surface treatment, by which a hard coat made of electrode material or a hard coat made of electrode material, which has reacted by electric discharge energy, is formed on a surface of a workpiece by energy of electric discharge when the electric discharge is generated between the electrode for electric discharge surface treatment and the workpiece.
Concerning the prior art by which a hard coat is formed on a surface of a workpiece so as to endow the workpiece with a corrosion resistance and abrasion resistance property, for example, Japanese Unexamined Patent Publication No. 5-148615 discloses a method of electric discharge surface treatment. According to the patent publication, there is provided a method of electric discharge surface treatment of metal including the steps of: conducting a primary processing (accumulation processing) in which a pressed powder electrode, which is an electrode for electric discharge surface treatment formed by compressing powder of WC (tungsten carbide) and Co (cobalt), is used; and conducting a secondary processing (remelting processing) in which the electrode is replaced with an electrode made of copper, the electrode consumption of which is relatively small. According to this method, it is possible to form a hard coat having a strong adhering force with respect to steel, however, it is impossible to form a hard coat having a strong adhering force with respect to sintered material such as cemented carbide.
However, according to the investigations made by the present inventors, the following knowledge has been found out. When an electrode for electric discharge surface treatment made of material such as Ti (titanium) capable of forming a hard carbide is used and electric discharge is generated between the electrode and metal of a workpiece, it is possible to form a strong hard coat on the metal surface of the workpiece without being subjected to the process of remelting. The reason why the strong hard coat is formed on the metal surface is that the electrode material, which has consumed by electric discharge, reacts with C (carbon) which is a component of processing solution, so that TiC (titanium carbide) can be created. Also, the following knowledge has been found out. When a pressed powder electrode, which is an electrode for electric discharge surface treatment made of metal hydride such as TiH2 (hydrogenation titanium), is used and an electric discharge is generated between the pressed powder electrode and the metal of a workpiece, it possible to form a hard coat more quickly, the adhering property of which is higher than that of a case in which Ti is used. Further, the following knowledge has been found out. When a pressed powder electrode, which is an electrode for electric discharge surface treatment in which hydrogenation compound such as TiH2 is mixed with other metal or ceramics, is used and an electric discharge is generated between the pressed powder electrode and the metal of a workpiece, it is possible to quickly form a hard coat of various hardness and abrasion resistance property.
The above method is disclosed, for example, in Japanese Unexamined Patent Publication No. 9-192937. FIG. 5 is an arrangement view showing an example of the device used for the electric discharge surface treatment described above. In FIG. 5, reference numeral 1 is a pressed powder electrode which is composed of compressed powder of TiH2, reference numeral 2 is a workpiece, reference numeral 3 is a processing tank, reference numeral 4 is a processing solution, reference numeral 5 is a switching element for switching a voltage and current impressed upon between the pressed powder electrode 1 and the workpiece 2, reference numeral 6 is a control means for controlling by turning on and off the switching element 5, reference numeral 7 is an electric power unit, reference numeral 8 is a resistor, and reference numeral 9 is a hard coat which has been formed. According to the above structure, when an electric discharge is generated between the pressed powder electrode 1 and the workpiece 2, the hard coat 9 can be formed on a surface of the workpiece 2 made of steel or cemented carbide by the electric discharge energy. In this structure, the switching element 5, control means 6, electric power unit 7 and resistor 8 correspond to an electric power unit for electric discharge surface treatment to determine a waveform of a pulse of an electric discharge current when the electric discharge surface treatment is conducted.
In the above conventional electric power unit for electric discharge surface treatment, an electric discharge current pulse, the waveform of which is rectangular, is basically used. As shown in FIG. 6, when peak value Ip of the electric discharge current and pulse width T are changed, the thickness of a coat formed on a workpiece is adjusted.
FIG. 7 is a photograph showing a cross section of the hard coat 9 formed on the workpiece 2 by electric discharge surface treatment in which the conventional electric discharge current pulse is used. On the photograph, it is possible to observe a state in which blow holes exist on the hard coat 9 and the material is not sufficiently melted in the process of forming the hard coat 9.
The following problems may be encountered in the conventional electric discharge surface treatment. As described above, since quick heating and quick cooling are repeatedly conducted in the electric discharge surface treatment in which the conventional electric discharge current pulse is used, blow holes exist on the hard coat formed on the workpiece, that is, it is impossible to form a tight hard coat. Further, residual stress remains on the surface which has been subjected to electric discharge surface treatment. Due to the influence of the residual stress, there is a possibility that cracks are caused on the surface which has been subjected to electric discharge surface treatment.
The present invention has been accomplished to solve the above problems. It is an object of the present invention to provide an electric power unit for electric discharge surface treatment and a method of electric discharge surface treatment capable of forming a tight hard coat on a workpiece without causing defects such as cracks on the surface of the workpiece.
The present invention provides an electric power unit for electric discharge surface treatment by which electric discharge is generated between an electrode for electric discharge surface treatment and a workpiece so that a hard coat is formed on a surface of the workpiece by energy of the electric discharge, the electric power unit for electric discharge surface treatment comprising: a control means for dividing an electric discharge current pulse into a first pulse width T1 (first peak value Ip1), a second pulse width T2 (second peak value Ip2), . . . , and an n-th pulse width Tn (n-th peak value Ipn) (n is an integer 2 and more), the control means for setting the k-th pulse width Tk and the k-th peak value Ipk (1xe2x89xa6kxe2x89xa6(nxe2x88x921), k is an integer) so that a quantity of supply of the hard coat material by the emission of the electrode material can be a predetermined value previously determined according to a predetermined processing condition, the control means for setting the n-th peak value Ipn so that the emission of the electrode material can be suppressed and the electrode component adhering to the workpiece can be melted, and the control means for setting the n-th pulse width Tn at the time corresponding to the predetermined melting condition of the hard coat.
The present invention also provides an electric power unit for electric discharge surface treatment by which electric discharge is generated between an electrode for electric discharge surface treatment and a workpiece so that a hard coat is formed on a surface of the workpiece by energy of the electric discharge, the electric power unit for electric discharge surface treatment comprising: a control means for setting an electric discharge current waveform in the substantial first half of the electric discharge current pulse width so that a quantity of supply of the hard coat material by the emission of the electrode material can be a value corresponding to a predetermined processing condition, and the control means for setting an electric discharge current waveform in the substantial second half of the electric discharge current pulse width so that the emission of the electrode material can be suppressed and an electrode component adhering to the workpiece can be melted according to a predetermined melting condition.
The present invention also provides a method of electric discharge surface treatment for forming a hard coat on a surface of a workpiece by which electric discharge is generated between an electrode for electric discharge surface treatment and the workpiece so that the hard coat is formed on the surface of the workpiece by the energy of electric discharge, the method of electric discharge surface treatment comprising the steps of: dividing an electric discharge current pulse into a first pulse width T1 (first peak value Ip1), a second pulse width T2 (second peak value Ip2), . . . , and an n-th pulse width Tn (n-th peak value Ipn) (n is an integer 2 and more) setting the k-th pulse width Tk and the k-th peak value Ipk (1xe2x89xa6kxe2x89xa6(nxe2x88x921), k is an integer) so that a quantity of supply of the hard coat material by the emission of the electrode material can be a predetermined value previously determined according to a predetermined processing condition; setting the n-th peak value Ipn so that the emission of the electrode material can be suppressed and the electrode component adhering to the workpiece can be melted; and setting the n-th pulse width Tn at the time corresponding to the melting condition of the hard coat.
The present invention also provides a method of electric discharge surface treatment for forming a hard coat on a surface of a workpiece by which electric discharge is generated between an electrode for electric discharge surface treatment and the workpiece so that the hard coat is formed on the surface of the workpiece by the energy of electric discharge, the method of electric discharge surface treatment comprising the steps of: setting an electric discharge current waveform in the substantial first half of the electric discharge current pulse width so that a quantity of supply of the hard coat material by the emission of the electrode material can be a value corresponding to a predetermined processing condition; and setting an electric discharge current waveform in the substantial second half of the electric discharge current pulse width so that the emission of the electrode material can be suppressed and an electrode component adhering to the workpiece can be melted according to a predetermined melting condition, so as to form the hard coat on the surface of the workpiece.
Since the present invention is composed as described above, the following effects can be provided by the present invention.
According to the electric power unit for electric discharge surface treatment and the method of electric discharge surface treatment of the present invention, the hard coat material can be appropriately supplied and melted. Therefore, it is possible to form a tight hard coat on a workpiece.
The hard coat formed on the workpiece is gradually cooled without being subjected to the repetition of quick heating and cooling. Therefore, it is possible to decrease defects such as cracks caused on a hard coat.